The fuel flow in diaphragm-type carburetors is dependent upon the pressure differential existing between the carburetor venturi and atmosphere. The venturi pressure depends upon engine design characteristics and operating conditions. A diaphragm-type carburetor generally comprises a mixture conduit in which fuel is mixed with air for delivery to the intake manifold of an engine, a fuel chamber closed by a diaphragm and communicating through a nozzle with the mixture conduit for delivery of fuel thereto, and valve means controlled by the diaphragm for controlling delivery of fuel from a fuel tank to the fuel chamber. An air filter is provided for cleaning air entering the mixture conduit. The mixture conduit is formed with a restriction, e.g., a venturi. With the engine in operation, air flows through the mixture conduit, a pressure drop occurs across the venturi (i.e. a partial vacuum is created in the venturi), and pressure on the outside of the diaphragm causes the diaphragm to flex inwardly and effect delivery of fuel through the nozzle, which is usually located at the throat of the venturi where the pressure drop is at maximum, and this diaphragm flexure effects opening of the valve means for delivery of fuel to the fuel chamber.
It is well known that intake tuning of engines often has an adverse affect on the fuel metering characteristics of the carburetor, particularly with respect to single cylinder engines operable over a relatively wide speed range. Engine intake tuning can cause the carburetor to deliver fuel in an incorrect ratio to the air flow due to unsteady air flow through the carburetor and to the effect of the moving pressure wave forms in the manifold and carburetor bore. These wave forms are created by the opening and closing of the engine intake valve(s) or port(s), and travel at the speed of sound, their behavior being well known in the art.
The effect of the moving pressure wave forms on the fuel delivered from the nozzle of the carburetor has long been a source of problems for the carburetor design engineer. The wave effect is superimposed on the normal vacuum caused by the engine intake air stream flow through the venturi. This in turn causes the nozzle to deliver fuel in a manner which is not fully responsive to the vacuum caused by the air flow. The carburetor will function properly when the pressure drop P across the main jet (some distance from the nozzle outlet in the venturi) is proportional to the density and the square of the velocity of the air flow, i.e., ##EQU1## The tuning waves are superimposed on the venturi pressure drop and adversely effect fuel metering otherwise designed to follow this relationship. More particularly, this tuning wave is imposed on the fuel delivery side of the main jet and adversely affects the desired design value of the pressure drop .DELTA. P, across the main nozzle-fuel controlling restriction.
It is believed that such tuning waves, i.e., air pressure waves generated in the carburetor mixture conduit by the sudden opening and closing of the engine intake port, are responsible for such well known problems as fuel "spit back" under certain engine operating conditions as well as certain abnormal and less well recognized deviations in the desired engine-carburetor performance curves plotting fuel-air ratio against engine speed, (e.g., undesirable "rich or lean spots" in the performance curves) and related plots of such parameters as specific fuel consumption, engine power output, exhaust constituents, etc.
Another well known problem adversely affecting the desired or design fuel metering characteristics of a carburetor, whether of the diaphragm or float type, is the gradual clogging by dirt, dust and/or other solid air borne particles of the engine air intake filter customarily disposed in front of the air entrance to the carburetor mixture conduit.